What is the Best Mix Ever?

[pensacola]

I am not a tech-diver, but was curious about something:

Let's say I was a maximally certified trimix tech diver that owned a scuba shop and could get any gas mix that I wanted totally free (would be nice, huh).

What mix would you use for the following scenario:

1st dive- 120 fsw max depth, maximum safe no deco bottom time

one hour surface interval

2nd dive- 80 fsw max depth, maximum safe no deco bottom time.

Both dives are flat wrecks that rise only 2 feet off the bottom, so it's a really flat profile. There is an anchor line for the ascent, but you can only carry the one bottle of gas on your back-- no switching. You will make a 4 min stop at 45 fsw and another 4 min stop at 15 fsw. You will have a normal ascent rate, but want to pick your gas properly in case you had to make an emergency ascent.

The way I currently dive, I would just use EAN 28 for dive #1 and EAN 34 for dive #2 and let the computer work its' magic. Just curious to see what the options would be if you were using trimix, and whether you would get more bottom time or safety or both.

 

[scubawally]

well when you weigh out the cost of the gas (He) and its decompression properties along with the cooling effect breathing He caused the body to go through.. for these relatively 'light' profiles and a managable END of less than 120' and a MOD of 120. plus the fact that you said no deco gas I would not recommend the use of helium the optimal gas mixes are what you have stated or somewhere clase to that. If you wanted to use only one gas it would have to be EAN28

 

[amascuba]

I am not a tech-diver, but was curious about something:

Let's say I was a maximally certified trimix tech diver that owned a scuba shop and could get any gas mix that I wanted totally free (would be nice, huh).

What mix would you use for the following scenario:

1st dive- 120 fsw max depth, maximum safe no deco bottom time

one hour surface interval

2nd dive- 80 fsw max depth, maximum safe no deco bottom time.

Both dives are flat wrecks that rise only 2 feet off the bottom, so it's a really flat profile. There is an anchor line for the ascent, but you can only carry the one bottle of gas on your back-- no switching. You will make a 4 min stop at 45 fsw and another 4 min stop at 15 fsw. You will have a normal ascent rate, but want to pick your gas properly in case you had to make an emergency ascent.

The way I currently dive, I would just use EAN 28 for dive #1 and EAN 34 for dive #2 and let the computer work its' magic. Just curious to see what the options would be if you were using trimix, and whether you would get more bottom time or safety or both.

Planning a trip the Oriskany? Helium isn't really used to extend bottom time. It's used to reduce the affects narcosis.

Making a 4min. stop at 45fsw won't help with off gassing if your scenario.

You would want to keep the PPO₂ below 1.4ATA. For 120fsw that would be a EAN30 mix. DIR standards would call for 30/30 for dives to this depth.

For dives to 80fsw I just use EAN32.

 

[omar]

I do a lot of diving in that range with my wife and we use 21/35. I tried 30/30 and other mixes (EANx) that work okay. But the 21/35 with appropriate slow ascents and stops at 30/20/10 (none of this 5@15 BS) seems to leave us feeling a lot better with no fatigue at the end of the day. Also the clarity and ease of breathing that mix at depth is noticeable.

Now for a little heresy.. You do not need to be Tmx certified to use it (my wife isn’t). You do need to understand deco and the physics behind it though. In other words you need to be a thinking diver. The early users of Tmx were not certified and there used to be lively discussions on Techdiver about the need for certification. Now that the agencies have created certifications for it, it has almost become mandatory.:no

I have the advantage of having my own setup and a business use that allows me to expense the gas. I don’t dive on air very often.

 

[omar]

well when you weigh out the cost of the gas (He) and its decompression properties along with the cooling effect breathing He caused the body to go through..

You need to acccount for the heat capacity and mass of the gas when calculting heat transfer. There is little to no difference in heat loss of this mix when compared to air or EANx.

 

[daniel f aleman]

You can use air for both dives, but I would increase SI to 2 hours. If I were to make your dive plan, I would go with straight EANx 28 for each dive.

Safe diving there...

 

[pensacola]

I have the advantage of having my own setup and a business use that allows me to expense the gas. I don’t dive on air very often.

That's what I was wondering-- are we diving EAN because it is simple and (relatively) affordable, or is it because it is the best for these type dives? I have been to the Oriskany several times, and have no desire to go below the flight deck (130 fsw). There is plenty to see at lesser depths. We commonly make trips to 120 fsw small low profile wrecks in this part of the gulf and I was wondering if it is worth the training to be able to use trimix for these.

 

[amascuba]

That's what I was wondering-- are we diving EAN because it is simple and (relatively) affordable, or is it because it is the best for these type dives? I have been to the Oriskany several times, and have no desire to go below the flight deck (130 fsw). There is plenty to see at lesser depths. We commonly make trips to 120 fsw small low profile wrecks in this part of the gulf and I was wondering if it is worth the training to be able to use trimix for these.

The only advantage that helium will add for recreational dives to 120fsw is help reduce narcosis. It wouldn't extend your NDL any. Going for the additional training would really be a personal choice of what you want to accomplish within diving. EAN28 is a perfectly fine gas within recreational limits on the Oriskany.

 

[omar]

It was a question of best gas regardless of cost. In my experience the use of helium is a better choice for any dive greater than 100 ft.

 

[pensacola]

I don't feel narcosis effects at 120' (maybe I'm not that susceptible or just unaware). Which gas would put me at less risk of DS (assuming the same dive profile)?

A. 21/35
B. EAN 28
C. It's a tie